The present invention relates to a novel halogenated aromatic compound, a polyphenylene polymer obtained by the polymerization of such a halogenated aromatic compound as a monomer component, and a proton-conductive membrane comprising a sulfonation product of such a polymer. It is known that a proton-conductive membrane can be used as a proton-conductive membrane for primary battery electrolyte, secondary battery electrolyte, fuel cell polymer solid electrolyte, display element, various sensors, signal transfer medium, solid capacitor, ion exchange membrane, etc.
Electrolytes are usually used as (aqueous) solutions in many cases. In recent years, however, there is a growing tendency to replace such aqueous soluble-form electrolytes with solid electrolytes. The first reason for this is the easiness of processing in applications of solid electrolytes to, e.g., the electrical/electronic materials mentioned above. The second reason is the trend toward reduction in weight, thickness, length and size, and toward energy saving.
Conventional proton-conductive materials include both inorganic materials and organic materials. Examples of the inorganic materials include uranyl phosphates which form hydrate. However, these inorganic compounds are insufficient in interfacial contact to pose many problems concerning the formation of a conductive layer on a substrate or electrode.
On the other hand, examples of the organic compounds include organic polymers such as polymers belonging to the so-called cation-exchange resins, e.g., sulfonated vinyl polymers such as sulfonated polymers with perfluoroalkylsulfonic acid represented by Nafion (manufactured by E. I. Du Pont de Nemours and Co., Inc.), and perfluoroalkylcarboxylic acid polymers, and polymers prepared with incorporating sulfonic acid groups or phosphoric acid groups into heat-resistant polymers such as polybenzimidazole and poly(ether ether ketone)s [see Polymer Preprints, Japan, Vol. 42, No. 7, pp. 2490-2492 (1993); Polymer Preprints, Japan, Vol. 43, No. 3, pp. 735-736 (1994); and Polymer Preprints, Japan, Vol. 42, No. 3, p. 730 (1993)].
These organic polymers are usually used in the form of a membrane. A conductive membrane made of these organic polymers can be bonded to an electrode while taking advantage of the solvent solubility or thermoplasticity. However, many of these organic polymers have the following problems besides being still insufficient in proton conductivity. The organic polymers deteriorate in durability or in proton conductivity at elevated temperatures (100xc2x0 C. or higher). When sulfonated, the organic polymers undergo embrittlement, deteriorate in mechanical strength and have a great dependence on humidity conditions. Further, the adhesion of the organic polymers to the electrode is not fully satisfactory. Moreover, the conductive membrane swells excessively during operation due to the hydrophilic polymer structure, and this swelling leads to a decrease in strength properties or a deformation. Consequently, application of those organic polymers to the aforementioned electrical/electronic materials and the like pose various problems.
U.S. Pat. No. 5,403,675 proposes a solid polymer electrolyte comprising a sulfonated rigid polyphenylene. This polymer is produced from a polymer comprising a phenylene chain obtained by polymerizing an aromatic compound (the polymer structure is described at column 9 in the patent specification) by reacting the phenylene polymer as the main component with a sulfonating agent to incorporate sulfonic acid groups thereinto. However, the incorporation of a large amount of sulfonic acid groups results in a sulfonated polymer having considerable deterioration in mechanical properties such as toughness (e.g., elongation at break, flexing resistance) and hot water resistance although proton conductivity improves with the increasing amount of sulfonic acid groups incorporated.
Accordingly one object of the invention is to provide a polymer which has a flexible structure in its main chain and thus exhibits a high toughness and can difficultly be deteriorated in its mechanical properties and thermal properties even when sulfonated.
Another object of the invention is to provide a sulfonic acid group-containing polymer obtained by the sulfonation of the polymer.
Still another object of the invention is to provide a proton-conductive membrane having an excellent mechanical strength and durability made of the sulfonic acid group-containing polymer.
The above objects of the invention will become apparent from the following detailed description and examples.
The invention provides a compound useful as a monomer effective for the incorporation of a flexible structure in a polymer. The compound is a halogenated aromatic compound represented by the following general formula (1m): 
wherein A independently represents an electron-withdrawing group; B independently represents an electron-donating atom or group; X represents a chlorine atom, iodine atom or bromine atom; R1 to R8 may be the same or different and each represent a hydrogen atom, fluorine atom or alkyl group; and n represents an integer of 2 or more, preferably 2 to 100, more preferably 2 to 80.
The halogenated aromatic compound provides a polymer with a flexible structure to enhance the toughness of the polymer.
The invention also provides a polyphenylene polymer having a repeating unit represented by the following general formula (1): 
wherein A, B, R1 to R8, and n are the same as defined in above.
The polyphenylene polymer may be a homopolymer or a copolymer containing other repeating units.
The invention further provides a polyphenylene copolymer having a repeating unit represented by the general formula (1) and a repeating unit comprising other divalent aromatic groups.
The invention further provides as one of the foregoing copolymers a polyphenylene copolymer wherein the repeating unit comprising a divalent aromatic group is at least one unit selected from the group consisting of those represented by the following general formulae (2) to (5): 
wherein A and B are the same as defined above; R9 to R15 may be the same or different and each represent a hydrogen atom or alkyl group; Z represents an aryl group; and m represents an integer of from 0 to 2; 
wherein R17 to R24 may be the same or different and each represent a hydrogen atom, fluorine atom, alkyl group or aryl group.
The polyphenylene copolymer can be easily sulfonated to provide proton conductivity.
The invention further provides the foregoing copolymer further containing a sulfonic acid group.
The sulfonic acid group-containing copolymer is useful as a material of proton-conductive membrane.
The invention further provides a proton-conductive membrane comprising the foregoing sulfonic acid group-containing copolymer.